The impact/fluidity (IF) value is defined as the ratio of the product (i) of the Izod notched impact at 23.degree. C. multiplied by the MFI (Melt Flow Index) of the composition including (A) and (B) to the product (ii) of the impact multiplied by the MFI of the same composition but not including (B).
This IF value can be obtained by a choice of the proportions of (A) and (B) and of the quantity of anhydride or acid in the copolymer (B), as will be explained later.
Improvement in the impact strength of polyamides is generally obtained by incorporation, in the form of a disperse phase, of an impact modifier which is elastomeric in character and has reactive functional groups (acrylic acid, maleic anhydride, etc.) capable of reacting with functional groups of the polyamide matrix. This reactivity makes it possible to ensure a fine and homogeneous dispersion of the elastomer, and good adhesion at the nodule/matrix interface, but results in a considerable drop in fluidity. This large change in the viscosity is detrimental to the processing, especially in the case of injection-moulded articles which are thin or large-sized.
The prior art has already described high-impact compositions based on polyamide.
EP 96 264 describes polyamides of viscosity between 2.5 and 5, reinforced with ethylene/C.sub.2 -C.sub.8 alkyl (meth)acrylate/unsaturated acid or anhydride copolymers including 0.5 to 8% by weight of acid or anhydride; the examples show only the content of 4%.
U.S. Pat. No. 5,070,145 describes polyamides reinforced with a mixture (i) of a polyethylene or of an ethylene/alkyl (meth)acrylate copolymer and (ii) of an ethylene/alkyl (meth)acrylate/maleic anhydride copolymer.
The copolymers employed contain 2.5 to 3% by weight of maleic anhydride.
U.S. Pat. No. 4,174,358 describes reinforced polyamides which are in the form of a polyamide matrix in which are dispersed nodules smaller than 1 .mu.m which have a certain modulus which must also be a fraction of the modulus of the polyamide. Very many reinforcing agents are described, some containing epoxide functional groups. Most are polymers which have neutralized acid or anhydride functional groups or are mixtures based on EPDM.
All this prior art has exemplified only reinforcing agents which have acid or anhydride functional groups or EPDMs, and no mention is made of the viscosity of the reinforced polyamides. In the comparative examples of the present invention it will be seen that the behaviour of the modifiers of the prior art has nothing to do with those of the present invention.
In the examples, only test pieces of small dimensions (4.times.6.times.50 mm) are produced for performing the impact strength measurements.
It is clear that with such dimensions and in laboratory conditions the viscosity is of no importance.
The situation is completely different in the case of articles which are thin or of larger dimensions or else of complex shape, when these articles must be produced at industrial rates.
EP 186 790 describes polyamides 6 modified with ethylene/n-butyl acrylate/maleic anhydride copolymers containing less anhydride than the prior art, to avoid a large drop in the fluidity. The examples show anhydride values of 0.22% or 0.7% by weight of maleic anhydride. The quantity of copolymer is either 8.7 or 25 parts per 100 parts of polyamide (that is to say 8 or 20% of the polyamide modified with the copolymer).